Statistical Analysis of Concurrent Electron Cyclotron Harmonic and Whistler Waves Driven by ULF Waves

Abstract

AbstractPlasma sheet electron precipitation to the diffuse aurora is one of the primary mechanisms of magnetospheric energy transport to the Earth’s atmosphere. Such precipitation is provided by electron resonant interactions with electron cyclotron harmonics (ECH) and whistler‐mode waves in the near‐Earth plasma sheet. ECH and whistler‐mode wave generation is traditionally attributed to two main sources of free energy: the loss‐cone anisotropy and the temperature anisotropy due to transverse electron heating during earthward convection or plasma sheet injections. This paper considers a third free energy source for ECH and whistler‐mode wave generation—the electron distribution modulation by compressional ultra‐low‐frequency (ULF) waves. We conduct statistics of ECH and whistler‐mode wave bursts associated with ULF waves and show that: (a) such bursts are mostly observed on the dawn flank, (b) 83% of events show a correlation between ECH and whistler‐mode bursts, and the rest 17% show anti‐correlation, (c) ECH and whistler‐mode wave intensities are comparable with those observed around the night‐side injection region. We investigate the main characteristics of ECH and whistler‐mode waves modulated by ULF waves and provide an empirical model for these characteristics. This model can be directly incorporated into simulation frameworks, so as to include these ULF‐modulated wave bursts in simulations of the magnetosphere‐ionosphere coupling and radiation belt dynamics.